My son started getting wooden train sets when he was 2. Over the next year he received enough pieces to never need another (I usually forbade anything with a face so that he would still use it after the "Thomas" age was over.) Depending on the friend, and if I move the wooden box we store it in around the house, the trains can still, 6 years later, be a major fascination. In fact, for years we had a stained wooden train table as our coffee table. If I had the trains in the box underneath, even adults would become fixated to see what type of maze of tracks they could create.
Also, because the kids have so loved trains, we have also gone to many model train shows. By going to the show, he sees VERY complicated ideas, people of all ages using trains as their own past time, and other to ask questions of. This semi-annual trek always sparks a new, passionate interest in the train set.
I have NEVER regretted letting trains come into our life.